1. Field of the Invention
The present invention relates to apparatus and method of measuring characteristics of a linear motor, and more particularly, to apparatus and method of measuring a thrust constant and a thrust ripple in a linear motor.
2. Description of the Related Art
An example of a linear motor will be briefly described with reference to FIGS. 1 and 2. In the linear motor, a fixed yoke 10 having an approximately U-shaped cross section is disposed on a base (not shown) so as to extend in a traveling direction. A moving body, for example, a coupling member 20, which is coupled with a stage, is disposed above the fixed yoke 10 so as to slide in the traveling direction. A movable coil section 21 is disposed under the coupling member 20. Permanent magnets 11 are fixed on the two inner surfaces of the fixed yoke 10 in the traveling direction at intervals, respectively, so as to face the two main surfaces of the movable coil section 21 through gaps.
Note that, in the case of a three-phase linear motor, the linear motor includes coils for three phases, that is, a U-phase, a V-phase, and a W-phase.
In the linear motor, electromagnetic force is generated by the mutual action between the magnetic flux from the permanent magnets 11 and the current flowing in the movable coil section 21 and causes the coupling member 20 and the movable coil section 21 to travel together.
The linear motor arranged as described above is used as a drive source of an X-Y stage apparatus in place of a ball thread mechanism because the linear motor can make positioning more accurately and generate a high thrust.
Incidentally, in this type of linear motors, a thrust constant and a thrust ripple are measured in a manufacturing process as factors for measuring characteristics thereof to know their performance.
A conventional method of measuring the thrust constant and the thrust ripple of a three-phase linear motor will be described with reference to FIG. 3. In FIG. 3, a load cell 41 is pressed against the movable section 31 (for example, the coupling member 20 in FIG. 1) of a linear motor as a subject to be measured. The load cell 41 is mounted on a stage 43 capable of measuring a moving distance by a micrometer 42. Then, the movable section 31 of the linear motor is pulled in a direction where it is pressed against the load cell 41 by a spring 44 such as a coil spring. A yoke 30 corresponds to the fixed yoke 10 in FIG. 1.
A given current is supplied to the coils of the respective phases (U-phase, V-phase, and W-phase) in the linear motor using the apparatus arranged as described above. At the time, a force necessary to prohibit the movement of the movable section 31 is repeatedly measured by the load cell 41 at the respective points of a plurality of permanent magnets disposed in a traveling direction. As a result, a thrust constant is obtained as an output signal from the load cell 41. In contrast, a thrust ripple is a difference (variable component) between the value obtained by subjecting the thrust constants of the respective phases to sine excitation, that is, the value obtained by combining the thrust constants of the three phases and a predetermined thrust.
FIG. 4A shows the thrust constants of the respective phases that are obtained as the output signals from the load cell 41. FIG. 4B shows the value obtained by combining the thrust constants of the three phases in FIG. 4A as the thrust ripple.
Note that the thrust constant and the thrust ripple may be measured by the following method in place of the method performed using the apparatus shown in FIG. 3.
With reference to FIG. 5, a linear motor as a subject to be measured is prepared as a slave linear motor 51 as well as a master linear motor 52 is used. The slave linear motor 51 is coupled with the master linear motor 52 through a contact type guide mechanism 53 such as a bearing mechanism.
The guide mechanism 53 can move along a guide rail 54.
When the master linear motor 52 is actuated, voltages are induced in the coils of the respective phases in the slave linear motor 51 in correspondence to the moving speed thereof, and the voltages (hereinafter, referred to as xe2x80x9cspeed-induced-voltagesxe2x80x9d) and the moving speed are measured. Then, a per-phase induced voltage constant is calculated from the ratio of the measured speed- induced-voltage of each phase and the moving speed. Further, the thrust constant and the thrust ripple are calculated from the thus obtained per-phase induced voltage constant of each phase.
In the method of measuring the thrust constant and the thrust ripple using the apparatus of FIG. 3, however, an error is included in the thrust constant and the thrust ripple due to the accuracy of a sensor such as the micrometer 42.
In contrast, the measuring method shown in FIG. 5 also lacks accuracy in measurement because the guide mechanism 53 is arranged as the contact type. Further, the thrust ripple cannot be accurately evaluated quantitatively because the drive system employed in the method is different from an actual drive system.
Accordingly, an object of the present invention is to improve a measuring accuracy in apparatus and method of measuring a thrust constant and a thrust ripple that determine the performance of a linear motor used to drive a stage.
A thrust ripple measuring apparatus according to the present invention is used for a linear motor. According to an aspect of the present invention, the apparatus comprises a master linear motor for driving a slave linear motor as a subject to be measured. A non-contact type stage mechanism couples the slave motor with the master motor. A voltage detector detects speed-induced-voltages generated in coils of the respective phases of the slave linear motor and a speed detector detects a moving speed of a movable portion in the slave linear motor when the master linear motor is actuated. A processing unit is supplied with the detected speed-induced-voltages and the detected moving speed and calculates a per-phase induced voltage constant from the ratio of the detected speed-induced-voltage of each phase and the detected moving speed of the slave linear motor. The processing unit further calculates a thrust constant and a thrust ripple from the calculated per-phase induced voltage constant.
According to a thrust ripple measuring method of the present invention, an apparatus is prepared in which a slave linear motor as a subject to be measured is coupled with a master linear motor for driving the slave linear motor through a non-contact type stage mechanism. When the master linear motor is actuated, speed-induced-voltages generated in coils of the respective phases of the slave linear motor are detected. At this time, a moving speed of the slave linear motor is detected. A per-phase induced voltage constant is calculated from the ratio of the detected speed-induced-voltage of each phase and the detected moving speed of the slave linear motor. A thrust constant and a thrust ripple is further calculated from the calculated per-phase induced voltage constant.